Pseudoisothermal ammonia process

ABSTRACT

Ammonia is produced in a reactor  22, 24, 246, 248, 250  or  252 , in which pseudoisothermal conditions can be approached by convective cooling of a reaction zone of the reactor by positioning at least a portion of the reaction zone in indirect contact with a flow of hot gas such as exhaust gas  18  or preheated air. The hot gas  18  may be supplied from a fired heater, a boiler  10 , a reformer  202 , a process air preheat furnace, a gas turbine, or the like. The reactor converts a feed stream of a purge gas  12  or syngas to ammonia. The method may be implemented in a primary synthesis loop (as at  246, 248, 250, 252 ) or in a purge gas loop  12  of a new ammonia plant, or by retrofitting an existing ammonia plant. Cooperatively installed with a primary ammonia synthesis loop, the reactor increases total ammonia production.

BACKGROUND OF THE INVENTION

This invention relates to a method for converting a feed stream ofnitrogen and hydrogen to ammonia in one or more ammonia synthesisreactors located in a flow of exhaust gas from a hot gas source toprovide heat transfer to the exhaust gas for pseudoisothermal operation.

Ammonia is commonly manufactured by reacting synthesis gas (syngas)components nitrogen and hydrogen in an ammonia synthesis loop includinga compressor, an ammonia synthesis reactor, ammonia condensation andrecovery units, and purge gas recovery. After a pass through the ammoniasynthesis reactor, the unreacted synthesis gas components are typicallyrecovered and recycled to the compressor and the reactor in a loop.Make-up synthesis gas is continuously added to the ammonia synthesisloop to provide fresh hydrogen and nitrogen.

Synthesis gas typically contains inert components introduced with themake-up syngas, including argon, methane, carbon dioxide, and others,that do not contribute to ammonia production and undesirably accumulatein the loop. Therefore, a purge gas stream is usually taken from theammonia synthesis loop to avoid an excessive concentration of the inertsin the loop. The purge stream is typically processed in a hydrogenrecovery unit, yielding a waste gas stream and a hydrogen-enrichedstream for recycle to the ammonia synthesis loop. The waste gas streamcomprises principally nitrogen with minor amounts of carbon dioxide,methane, hydrogen, and argon. In some cases, the waste gas can be usedas a low heating value fuel gas.

A significant technological advance in the manufacture of ammonia hasbeen the use of highly active synthesis catalysts comprising a platinumgroup metal such as ruthenium on a graphite-containing support, asdescribed in U.S. Pat. Nos. 4,055,628, 4,122,040 and 4,163,775. Also,reactors have been designed to use this more active catalyst, such as acatalytic reactor bed disclosed in U.S. Pat. No. 5,250,270. Otherammonia synthesis reactors include those disclosed in U.S. Pat. Nos.4,230,669, 4,696,799, and 4,735,780.

Ammonia synthesis schemes have also been developed based on the highlyactive synthesis catalyst. U.S. Pat. No. 4,568,530 discloses reacting astoichiometrically hydrogen-lean synthesis gas in an ammonia synthesisreactor containing a highly active catalyst in the synthesis loop.

U.S. Pat. No. 4,568,532 discloses an ammonia synthesis reactor, based ona highly active catalyst, installed in series in the ammonia synthesisloop downstream from a reactor containing a conventional iron-basedsynthesis catalyst.

U.S. Pat. No. 4,568,531 discloses introducing a purge stream from aprimary synthesis loop into a second synthesis loop using a more activesynthesis catalyst to produce additional ammonia from the purge stream.Another purge stream, significantly reduced in size, is taken from thesecond synthesis loop to avoid an excessive buildup of inerts. Thesecond synthesis loop, like the primary ammonia synthesis loop, employsa recycle compressor to recycle synthesis gas to the active catalystreactors in the second synthesis loop.

U.S. Pat. No. 6,171,570 discloses converting hydrogen and nitrogen intoadditional ammonia from a purge stream from an ammonia synthesis loop,using an ammonia synthesis reactor that does not require staged cooling.In particular, ammonia synthesis loop purge gas is provided to an inletof a shell and tube reactor having an ammonia synthesis catalyst on thetube-side, while boiler feedwater (BFW) is supplied to the shell-side ofthe reactor to provide cooling and/or to generate steam.

U.S. Patent Application Publication 20030027096, Barnett et al.,describes a method to increase reforming furnace efficiency bypreheating a reagent stream and generating synthesis gas in catalyticreactors heated in radiant, transition, and convective sections of asteam-methane reforming furnace.

Patents and publications referred to herein are hereby incorporated byreference in their entireties.

SUMMARY

This invention relates to the production of ammonia from synthesis gasin at least one reactor while maintaining pseudoisothermal conditionsusing preheated air or exhaust gas as the cooling medium to carry awaythe exothermic heat of the ammonia synthesis reaction and heat the airor exhaust gas. The cooling is achieved by positioning at least aportion of a reaction zone in an exhaust duct or air preheat duct. Thehot gas may be supplied from a variety of process equipment including afired heater, a boiler, a reformer, a process air preheat furnace(including preheated air or flue gas), a gas turbine, or the like. Theinvention can be implemented in a new ammonia plant or by retrofittingan existing ammonia plant. It can also be used in a primary ammoniasynthesis loop or in secondary synthesis from a purge gas stream.

This invention provides features and capabilities not available togetherin other methods: (a) an ammonia syngas or synthesis purge gas syngas isreacted so as to maximize productivity and minimize demand for makeupsyngas per unit output of ammonia; (b) a pseudoisothermal condition inthe synthesis reactor maximizes synthesis conversion; (c) thepseudoisothermal condition in the reactor minimizes space velocities,which help to maximize efficiency of catalyst utilization; (d)converting purge gas to additional ammonia minimizes the amount of wastegas and the size of waste gas processing equipment, and/or debottleneckswaste gas treatment where this is a limitation on ammonia production;(e) converting purge gas at pressures near primary synthesis pressuresfacilitates recycling unreacted effluent to reforming; and/or (f) aplurality of high-pressure, heat-generating synthesis reactors andheat-consuming recovery units can be arrayed in an exhaust gas streamwherein heat duties can be interchanged in a simplified mechanicaldesign and potential process stream leakage poses substantiallyminimized risk of cross-contamination.

In one embodiment the present invention provides a process forconverting a feed stream of nitrogen and hydrogen to ammonia. Theprocess includes supplying the feed stream to an inlet of apseudoisothermal ammonia synthesis reactor having a reaction zonecomprising a plurality of catalyst tubes positioned in indirect heatexchange relationship with a gas stream in a hot gas duct of acombustion unit to maintain the reaction zone between 300° C. and 650°C. The nitrogen and hydrogen are reacted in the reaction zone to form aproduct stream having an increased ammonia content relative to the feedstream. The product stream is recovered from an outlet of the reactor.

A temperature rise between the inlet and outlet is desirably less than80° C., more desirably less than 55° C. The reaction zone is desirablymaintained at a temperature from 370° C. to 480° C. The catalyst tubescan have an extended surface. The feed stream can comprise from 50 to 75volume percent hydrogen and from 25 to 40 volume percent nitrogen.

The feed stream can be purge gas from a primary ammonia synthesis loop.The process can also include water washing the product gas stream toremove ammonia and recover an ammonia-lean stream, and injecting theammonia-lean stream into feed to a reformer, desirably upstream of amixed feed coil in a primary reformer. The process can further includedehydrating the purge gas from the primary ammonia synthesis loop,supplying a first portion of the dehydrated purge gas as the feed streamto the ammonia synthesis reactor, supplying a second portion of theammonia-lean gas stream to hydrogen recovery to obtain a hydrogen-richstream, and recycling the hydrogen-rich stream to the primary ammoniasynthesis loop.

The catalyst can be magnetite or a high activity catalyst such as aplatinum-group metal. The combustion unit can be a fired heater, boiler,reformer, air preheat furnace, or gas turbine. The hot gas can beexhaust from any of these combustion units, or preheated air from an airpreheat furnace.

The process can also include heating a process stream in a heat recoveryunit in communication with the hot gas duct to contact the hot gasdownstream from the ammonia synthesis reactor. The process can includearraying a plurality of synthesis reactors and a plurality of heatrecovery units in communication with the hot gas duct to alternatinglyin series indirectly reject and recover heat from the synthesis reactorsand heat recovery units, respectively, using the hot gas as a commonheat transfer medium.

In another embodiment, the invention provides a pseudoisothermalsynthesis unit for converting nitrogen and hydrogen in a feed stream toammonia. The synthesis unit includes means for supplying the feed streamto an inlet of a pseudoisothermal ammonia synthesis reactor having areaction zone comprising a plurality of catalyst tubes positioned inindirect heat exchange relationship with a gas stream in a hot gas ductof a combustion unit to maintain the reaction zone between 300° C. and650° C. Means are provided for reacting the nitrogen and hydrogen in thereaction zone to form a product stream having an increased ammoniacontent relative to the feed stream. The synthesis unit also includesmeans for recovering the product stream from an outlet of the reactor.

Another embodiment of the invention provides a pseudoisothermalsynthesis unit for converting nitrogen and hydrogen in a feed stream toammonia. The unit includes an ammonia synthesis reactor having areaction zone to convert the feed stream to ammonia, an inlet tointroduce the feed stream into the reactor, a combustion unit having ahot gas duct in communication with the reaction zone to contact thereaction zone with a hot gas stream to indirectly transfer heat with thereaction zone, and an outlet to discharge a product stream from thereactor. The reaction zone can include a plurality of reactor tubes andat least one catalyst, which can be selected from the group consistingof magnetite, platinum-group metals, combinations thereof, and the like.The combustion unit can be selected from a fired heater, boiler,reformer, process air preheat furnace, or gas turbine. The synthesisunit can have an array including a plurality of ammonia synthesisreactors and heat recovery units in communication with the hot gas ductto alternatingly in series reject and recover heat indirectly betweenreactors and heat recovery units, respectively. An ammonia recoverysystem can be connected to the outlet including a product cooler, aproduct scrubber, and a scrubbing liquor stripper.

Another embodiment provides a method for converting an original ammoniaplant to a converted ammonia plant. The method is applicable to originalammonia plants having a primary synthesis loop with a primary ammoniasynthesis converter for converting synthesis gas to ammonia, and anammonia recovery section for separating ammonia vapor from a purge gasfrom the primary synthesis loop comprising hydrogen, nitrogen andinerts. A secondary ammonia synthesis loop is installed for reacting afeed stream to form ammonia including an ammonia synthesis reactorcomprising catalyst and having a respective reaction zone incommunication with a hot gas duct of a combustion unit to contact a hotgas stream as a heat transfer medium. A portion of the purge gas isdiverted to the secondary ammonia synthesis loop to form a secondaryammonia product stream. The secondary ammonia synthesis loop can includea cooler and a condenser to separate an ammonia-rich stream from thesecondary ammonia product stream and form a residual gas stream forrecycle to reformer feed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic for the conversion of hydrogen and nitrogen inparallel reactors positioned in series in the flow of an exhaust gasstream from a package boiler.

FIG. 2 is an expanded view of section 2 in FIG. 1.

FIG. 3 is a block diagram of a primary synthesis loop configured with asecondary synthesis loop.

FIG. 4 is a schematic of secondary ammonia synthesis from a purge gas.

FIG. 5 is a schematic for an ammonia plant with ammonia converterscooled using flue gas from the reformer.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides a process to convert nitrogen andhydrogen to ammonia. An exothermic catalytic reactor is placed in a hotgas duct of a combustion unit, for example a gas turbine, packageboiler, air preheater, primary reformer, or any other fired heater orequipment that may be available. Heat is transferred from the ammoniareactor to heat the hot gas, such that pseudoisothermal reactionconditions can be approached in the reactor, for example the temperatureincrease of the reactants between the inlet and outlet of the reactorcan be limited to less than 100° C.

The feed stream of hydrogen and nitrogen is supplied to an inlet of atleast one ammonia synthesis reactor comprising a plurality ofcatalyst-containing reactor tubes. The feed stream passes through thesynthesis reactor tubes, to form a product gas having increased ammoniacontent relative to the feed stream. The synthesis reaction isexothermic. The reactor tubes penetrate the hot gas duct such that thehot gas flows across the reactor tubes, dissipating reaction heat intothe hot gas and maintaining pseudoisothermal reactor conditions. Theheat imparted to the combustion unit gas can be recovered by heatingboiler feed water (BFW) for steam generation, preheating combustion airor a feed stream to a synthesis gas reactor, heating a process stream,or the like, using heat recovery equipment typically found in hot gasducts associated with the combustion unit.

By operating at a nearly constant temperature, the reaction has a closerapproach to equilibrium, which in turn requires less catalyst for thereaction. In addition, the dissipation of heat decreases the chances forhot spots in the reactor and prolongs catalyst life.

Additionally, unlike conventional shell-and-tube synthesis reactorsystems, the present method allows any leaks from duct-installedsynthesis reactor tubes, steam coils, and process heat exchangers or BFWcoils to pass into the exhaust gas and be controlled or vented. Thissubstantially minimizes any risk of cross-contamination between processstreams. Moreover, since boiler feedwater at elevated pressure is notused as a reactor-cooling medium in contact with the reactor tubes,there is minimal risk of catalyst poisoning from the BFW in the event ofa breach of a reactor tube wall.

The feed to the ammonia synthesis reactor can comprise a streamincluding nitrogen and hydrogen at reactable concentrations, such as asynthesis gas, recycle syngas, or ammonia synthesis loop purge gas.

The catalyst used in the ammonia synthesis reactor can be a conventionalammonia conversion catalyst such as magnetite. Additionally, ahigh-activity catalyst can be used, such as a catalyst of group VIII, orthe platinum group metals, such as ruthenium.

Pseudoisothermal ammonia conversion can be used in a secondary synthesisloop of an ammonia synthesis unit to form ammonia from a purge gasstream from the primary loop. Ammonia production is thereby maximizedand waste gas rejection is minimized. Alternatively, pseudoisothermalammonia conversion can be utilized in a primary ammonia synthesis loop.A plurality of ammonia synthesis reactors can be used in combination,comprising one or more catalysts. For example, a synthesis reactor usinga high-activity catalyst can be configured downstream of a reactorcontaining magnetite catalyst. The magnetite-containing reactor acts asa guard bed for the high-activity catalyst in the downstream reactor. Asa result, the high-activity catalyst can be used in a relatively coarsesize form, particularly to reduce dynamic pressure drop in the synthesisloop.

As one example, ammonia synthesis reactors can be disposed in theconvection section of a hydrocarbon reforming furnace, alone or incombination with a syngas pre-reformer. The pre-reformer, desirablydisposed in a transition section of the reforming furnace as describedin patent application publication U.S. 20030027096, Feb. 6, 2003,Barnett, et al., which is hereby incorporated herein by reference in itsentirety, partially cools the exhaust gas through the transitionsection. At least one synthesis reactor in communication with thereformer convection section further cools the partially cooled exhaustgas leaving the transition section

Generally, initial design of a plant with a primary ammonia synthesisloop can be configured in cooperative combination with secondarysynthesis reaction of the present invention. Secondary synthesis isdesirably applied in a purge gas loop to further convert residualnitrogen and hydrogen to additional ammonia. The design methodology ofthis arrangement is also advantageously applied in the retrofit of anexisting ammonia plant having only a primary synthesis loop, or toreplace a poorly performing secondary synthesis loop reactor.

In one embodiment as shown in the package boiler 10 seen in FIG. 1, apurge gas feed stream 12 containing nitrogen and hydrogen is heated inexchanger 14 mounted in an exhaust duct 16 for the boiler exhaust gas18. Preheated feed stream 20 is then fed to catalyst-containing tubes inparallel ammonia synthesis reactors 22, 24. Reactor effluent 26 flowsdownstream to conventional ammonia recovery (not shown). Boilerfeedwater (BFW) is supplied through line 28 successively to BFW heatingunits 30, 32, and 34. BFW heaters 30, 32 can be positioned downstreamfrom the respective reactors 24, 22 to recover heat from the exhaust gasflow 18 at elevated temperatures. BFW heater 32 thus functions as aninterstage cooler between the upstream and downstream reactors 22, 24 sothat the temperature conditions, flow rates and conversion rates in theparallel reactors 22, 24 can be essentially equivalent. Cooled exhaustgas 36 flows downstream from the BFW heater 30 for conventionalprocessing.

Reactions in the purge gas synthesis reactors 22, 24 can be constrainedto pseudoisothermal conditions by heating the exhaust gas 18 to removethe heat of reaction. The exhaust gas 18 serves as a common heattransfer medium, successively alternating between heat removal fromexothermic reactors 22, 24 and heat recovery to the exchangers 14, 30,32, 34.

The reactors 22, 24 can be designed to specific applications andpurposes by taking into account the flow rates of syngas 12 and exhaustgas 18, tube surface area, heat transfer coefficients, stream residencetimes, dynamic pressure losses, conversion rates, and like designfactors. The pseudoisothermal temperature rise (ΔT) in the syngas 12 isdesirably less than 80° C., and more desirably less than 50° C. Thelimits of operating temperature in the synthesis reactors 24 are ingeneral from 300° to 650° C., and desirably from 370° to 480° C. Theexhaust gas 18 can have a temperature less than the desired reactiontemperature, but the temperature should not be so low that the reactiontemperature anywhere in the reactor is less than the syngas feedtemperature, taking into account the flow rates of each.Pseudoisothermal conditions and startup can be facilitated by using hotgas for reactor cooling at a minimum temperature of 300° C.

As nitrogen and hydrogen in the feed stream 12 are converted in thereactor tubes 22, 24, ammonia concentration in the stream increases. Thepurge gas feed stream can have ammonia concentrations in a range of upto 10 volume percent, and the product stream 26 from 10 to 40 volumepercent.

The present invention avoids undesirable mechanical design elements seenin conventional synthesis reactors. The present design is rather simplein contrast to conventional ammonia synthesis reactors typicallyembodying a complex design as a shell-and-tube exchanger whereinsynthesis gas passes shell side sequentially through multiple radialand/or axial flow reactor stages housed in a high-pressure vessel forpreheating the syngas and interstage cooling of intermediate reactoreffluents. In contrast to isothermal operation with boiler feed water asa heat transfer medium employing elevated pressures, such as, forexample, from 6.8 to 10.3 MPa, the present invention can use inexpensivelow-pressure vessel designs for the heat removal media.

FIG. 2 shows an enlarged vertical arrangement of reactor tubes 23disposed in two transverse rows within the synthesis reactors 22, 24installed in the exhaust gas duct 16 of FIG. 1. The number of tubes 23depends on the desired tube size and design throughput rates of syngas20. The tubes 23 may be oriented vertically or horizontally, or atoblique angles. In the illustrated embodiment, the tubes 23 are orientedvertically to facilitate catalyst loading and removal. Inlet manifolds38 distribute the syngas feed stream 20 from a common header into thecatalyst-filled tubes 23. Outlet manifold 40 gathers the ammonia-richeffluent exiting the catalyst tubes 23 into product stream 26.

The outlet manifold 40 can support the tubes 23 at lower ends thereof.The outlet manifold 40 can in turn be supported by structural members(not shown) on either side of the ammonia synthesis reactors 22, 24. Itis desirable to orient the reactor tubes transversely, e.g.perpendicularly, with respect to the flow of the exhaust gas 18 throughthe exhaust duct 16 to maximize heat transfer coefficients and improvetemperature differences between the syngas and the exhaust gas.

FIG. 3 is a schematic for an ammonia plant 100 incorporating secondaryammonia synthesis 105 integrated with a primary ammonia synthesis loop110. The primary ammonia loop 110 includes syngas compression 122,primary ammonia synthesis 124, ammonia condensation and purification126, ammonia recovery 128, and hydrogen recovery 130, all of which aregenerally well known in the art. Briefly, a makeup syngas stream 132 ofnitrogen and hydrogen has a purity from about 95 to 100 volume percent,typically from 97.5 to 99.5 volume percent. Compression 122 supplies themakeup syngas 132 and recirculated syngas 138 at a suitable pressure forammonia synthesis. Syngas stream 140 is directed to primary ammoniasynthesis 124, and ammonia-rich product gas 142 flows to unit 126 fornearly isobaric stagewise refrigeration and condensation. Ammonia-leansyngas vapor 138 is recirculated to compression 122 as previouslymentioned, and a slipstream 144 of the ammonia-lean syngas vapor isdiverted to high-pressure ammonia recovery 128 to separate water vaporand noncondensable gases. Condensate formed in equilibrium with therecycle vapor 138 can be used as makeup refrigerant in thecondensation/purification unit 126. The refrigerant cyclically condensesand flashes through a plurality of stages (not shown) withincondensation/purification 126, yielding a purified ammonia stream 146,in a manner well known in the art.

A slipstream 148 of partially purified ammonia refrigerant is divertedto ammonia recovery 128 for use as makeup liquid to ammoniadistillation. A flashed refrigerant slip stream 150 comprisinglow-pressure ammonia plus noncondensable gases and other vapor from therefrigeration can be diverted to ammonia recovery 128 to separate watervapor and noncondensable gases. Ammonia recovery 128 returns anupgraded, low-pressure ammonia vapor stream 152 to the refrigerationsubsystem. Ammonia recovery 128 produces a low-pressure waste gas stream154, typically at a low mass flow rate of about 0.1 to 0.5 percent ofthe mass flow rate of makeup syngas 132.

A high-pressure purge gas stream 156 is taken from ammonia purification128 to remove inert gases such as argon, carbon dioxide, and methanethat accumulate in the primary synthesis loop. A portion 157 of thepurge gas 156 is sent to conventional hydrogen recovery 130. Thehydrogen is recovered as low-pressure hydrogen stream 134 andhigh-pressure hydrogen stream 136 that can be recycled with the syngasto compression 122 and ammonia synthesis 124. Waste gas comprisingprimarily nitrogen, plus argon, carbon dioxide, and methane in minorproportions flows through line 158 and together with waste gas stream154 to stream 160.

Another portion of the purge gas 156 is supplied as a feed 12 tosecondary synthesis 105, which includes a pseudoisothermal converter inpackage boiler unit 10, as described above in reference to FIGS. 1 and2, that produces an ammonia-rich effluent for feed to ammonia recoveryunit 164, which is described in more detail below in reference to FIG.4. The secondary recovery 164 imports partially purified ammoniarefrigerant 166 from condensation/purification 126 as makeup liquid forammonia distillation, and returns a high-concentration ammonia vaporstream 168 to stream 152. Ammonia-lean stream 170 comprises nitrogen andhydrogen and at high pressure, and if desired can be recycled toreformer feed, desirably upstream of a mixed-feed preheat coil.

In operation, the secondary synthesis improves plant productivity by:(1) increasing ammonia production, (2) reducing syngas makeup demand,and (3) reducing purge gas losses. Ammonia conversion in the secondarysynthesis can be from about 5 to 20 percent, for example 10 to 15percent, of the feed 12.

The purge gas stream 157 in a primary ammonia synthesis loop withoutsecondary ammonia conversion typically has a mass flow rate equivalentto about 15 to 25 percent of the mass flow rate of the makeup syngas132. In contrast, purge gas flowrates obtained by implementing thesecondary synthesis of this invention can be reduced in a range of 35 to65 percent, desirably by about 50 percent. Waste gas 160 is reduced byup to 10 to 15 percent, desirably from 5 to 10 percent. Hydrogenrecovery rates via recycle streams 134, 136 remain at about 60 to 80percent of the hydrogen in the purge gas 157, usually about 70 to 75percent.

With reference to the embodiment shown in FIG. 4, high-pressure purgegas in stream 12 can be heated in cross-exchanger 174 for feed topseudoisothermal ammonia conversion in the exhaust duct of packageboiler 10 (see FIGS. 1-2). Ammonia-enriched effluent stream 26 is cooledin the cross-exchanger 174 and supplied via line 162 to high-pressurescrubber 176 for contact with lean aqueous ammonia liquor stream 178.Ammonia-rich liquor 179 from the scrubber 176 is reheated incross-exchanger 180, depressurized across valve 182, and fed todistillation column 184. Distillation column 184 is refluxed withpartially purified ammonia refrigerant via stream 166, and producesoverhead stream 168 comprising high-concentration ammonia vapor returnedto condensation/purification 126 (see FIG. 3). The bottoms can be cooledin cross-exchanger 180 and one or more exchangers 186 for recirculationvia line 178 to the scrubber 176. Ammonia-lean syngas 170 is dischargedoverhead from the scrubber 176.

As seen in the embodiment of FIG. 5, both primary and secondary ammoniasynthesis reactors can be disposed within the convection section of aflue gas exhaust duct 200 from the fired section of an otherwiseconventional steam reformer 202. As is well known in the art, a naturalgas stream 204 is passed through sulfur removal unit 206, mixed withsteam via line 208, preheated in mixed feed preheater 202A and fed to aplurality of catalyst-filled tubes 202B in the primary reformer 202. Theeffluent 209 is then fed with air 210 to a conventional secondaryreformer 212. The raw syngas is passed through heat recovery unit 214and low temperature shift converter 216, which can be convenientlydisposed in duct 200, to convert CO and water to form additionalhydrogen and CO2. Thence, the gas is preheated in exchanger 218 andpassed through high temperature shift converter 220 to form additionalhydrogen, then to heat recovery unit 224, CO₂ removal unit 226,preheater 228, and methanator 230 to form makeup syngas stream 232,which is pressurized in makeup compressor 234 and fed to recyclecompressor 236.

Syngas at loop pressure in line 238 is heated in cross-exchanger 240 andpreheater 242 disposed in duct 200, and fed to reactor 246 that cancontain magnetite catalyst. The reactor 246 is disposed in the duct 200for cooling by the flue gas medium. The partially converted effluentfrom reactor 246 is then passed serially though reactors 248, 250, 252containing high activity catalyst and similarly cooled by the flue gasmedium in duct 200. The ammonia-rich effluent 253 is then cooled in thecross exchanger 240 and refrigeration unit 254, and ammonia is recoveredfrom separation unit 256, essentially as described in reference to FIG.3 above.

A sidestream 258 is taken from line 253 and fed in part to hydrogenrecovery unit 260 and in part to secondary converter 262. Hydrogenrecovery unit 260 is operated with refrigeration from the separationunit 256 essentially as described in reference to FIG. 3, and recovers ahydrogen stream 264 from the purge stream which is recycled to thecompressor 236. A waste gas stream 265 is disposed of as described inFIG. 3 above. The secondary converter 262 is a once-through ammoniaconverter which can be placed in a hot gas duct of a combustion unitsuch as package boiler unit 10 (see FIG. 1) and/or duct 200 to producean ammonia enriched stream 266, which is fed to ammonia stripping unit268 to recover a concentrated ammonia stream 270 that can be processedin separation unit 256. An ammonia-lean syngas stream 272 can berecycled to the feed to the reformer 202 upstream from the mixed feedpreheater 202A.

EXAMPLE 1

Table 1 below provides exemplary tube specifications for the embodimentof the heat transfer units in FIGS. 1 and 2, including the synthesisreactors 22, 24, the BFW heating units 30, 32, 34, and the syngaspreheater unit 14. Typically, the inner diameter of catalyst-containingtubes 23 can range from about 7.5 cm to about 10.0 cm, while the outerdiameter desirably ranges from about 8.25 cm to about 10.8 cm. Thelength of the catalyst tubes 23 typically ranges from about 5.0 m toabout 8.0 m, depending upon the diameter or other transverse dimensionof the exhaust duct 16.

The transverse orientation of the reactor tubes 23 and a relatively highexhaust gas velocity through the exhaust duct 16 may provide a suitablyhigh convective heat transfer coefficient to allow the reactors 22, 24to use less costly smooth-walled reactor tubes 23. Alternatively, aslisted in Table 1, the reactor tubes 23 can use extended surfaces suchas fins to enhance heat transfer. TABLE 1 Heat Transfer Coil InformationPreheat BFW 1 Reactor 1 BFW 2 Reactor 2 BFW 3 Element ID 14 34 22 32 2430 Coil Material S/S 304 H Carbon S/S 304 H Carbon S/S 304 H CarbonSteel Steel Steel Tube OD (cm) 8.89 5.08 8.89 5.08 8.89 5.08 Min. Wall(cm) 0.665 0.318 0.665 0.318 0.665 0.318 Tubes per Row 11 16 11 16 11 16Tube Rows 4 2 12 1 12 12 Passes 11 16 132 16 132 16 Spacing, 6.5 × 5.634.5 × 3.9 6.5 × 5.63 4.5 × 3.9 6.5 × 5.63 4.5 × 3.9 Centers × Rows (cm)Fin Material S/S 410 Carbon S/S 410 Carbon S/S 410 Carbon Steel SteelSteel Fins/cm 1.58 2.37 1.58 2.37 1.58 2.37 Fin Height × 1.905 × 1.746 ×1.905 × 1.746 × 1.905 × 1.746 × Thickness 0.127 0.127 0.127 0.127 0.1270.127 (cm)

EXAMPLE 2

This example compares the performance of an ammonia plant using thesecondary synthesis loop in an embodiment of the present invention as inFIGS. 1-4, relative to performance of a stand-alone “base-case” ammoniaplant, without any secondary ammonia synthesis. Table 2 provides datafor key process streams indicating performance of the base-case plant.Table 3 provides data illustrating performance of an ammonia plant inwhich the ammonia production from the primary synthesis loop 110 issupplemented with the secondary synthesis 105. In the Table 3 process,stream 12 supplies a portion of the purge stream 156 to the secondarysynthesis in the modified package boiler unit 10. Without the secondarysynthesis 105 in the base-case system of Table 2, the whole purge stream156 is supplied to hydrogen recovery unit 130.

A comparison of Tables 2 and 3 shows that total ammonia production withsecondary conversion increases by about 5 percent over the base case,while waste gas flow is reduced by about 8 percent. The makeup syngasfeed increases compared to the base case, due to the recycle ofunreacted syngas from the secondary conversion into the mixed feed forreforming. TABLE 2 Base Case Ammonia Plant Without Secondary SynthesisSyngas Makeup LP H2 Reactor HP H2 Recycle Purge Split to 2° from 2° NH₃Waste Stream Description Syngas Recycle Product Recycle Gas FeedSynthesis Synthesis Product Gas Stream: 132 134 142 136 138 156 157 170146 160 Composition (Dry Mole %) H2 71.04 77.94 48.34 81.15 57.2 58.96 08.67 N2 27.77 20.68 27.88 17.3 32.98 33.99 0 71.47 CH4 0.85 0.55 3.930.74 4.64 4.76 0 14.27 AR 0.33 0.82 1.86 0.81 2.20 2.26 0 5.60 NH3 0 018 0 2.99 0 100 0 Dry Flow 9,860 114 29,307 1,217 22,798 1,897 4,594 584(kg-mol/h) Dry Flow (kg/h) 93,503 887 387,452 8,431 285,482 23,49678,232 14,427 H2O (kg-mol/h) 0.0 0.0 0.0 0.0 0.0 0.8 0.6 0.0 Total Flow9,860 114 29,307 1,217 22,798 1,898 4,594 584 (kg-mol/h) Total Flow(kg/h) 93,503 887 387,452 8,431 285,482 23,511 78,241 14,429 Temperature(° C.) 4 19 453 17 25 24 −33 23 Pressure (MPa) 3.58 3.65 8.86 8.49 8.588.52 0.39 0.34 Density (g/cm3) 0.015 0.012 0.019 0.023 0.042 0.041 0.680.004 Average MW 9.4 7.8 13.2 6.9 12.5 12.4 17.0 24.8

TABLE 3 Ammonia Plant With Secondary Synthesis Syngas Makeup LP H2Reactor HP H2 Recycle Purge Split to 2° from 2° NH₃ Waste StreamDescription Syngas Recycle Product Recycle Gas Feed Synthesis SynthesisProduct Gas Stream: 132 134 142 136 138 156 157 170 146 160 Composition(Dry Mole %) H2 69.60 78.40 48.7 80.35 57.78 59.55 59.55 56.06 0 8.95 N228.58 20.88 27.87 17.82 33.06 34.08 34.08 36.07 0 72.84 CH4 1.14 1.143.17 0.56 3.76 3.88 3.88 4.73 0 11.88 AR 0.68 0.68 2.05 0.93 2.43 2.52.5 3.06 0 6.32 NH3 0.0 0.0 18.2 0.34 2.99 0 0 0.07 100 0.01 Dry Flow11,819 108 29,100 1,167 20,788 1,787 1,834 1,500 4,840 528 (kg-mol/h)Dry Flow (kg/h) 116,562 857 384,630 8,269 259,999 22,106 22,680 19,84582,428 13,215 H2O (kg-mol/h) 3.0 0.0 0.0 0.0 0.0 0.0 0.0 7.7 0.6 0.0Total Flow 11,822 105 29,100 1,167 20,788 1,787 1,834 1,508 4,840 528(kg-mol/h) Total Flow (kg/h) 116,616 803 384,630 8,269 259,999 22,10622,680 19,984 82,437 13,215 Temperature (° C.) −1 17 455 17 −23 25 25 72−33 25 Pressure (MPa) 3.62 3.65 8.86 8.48 8.64 8.49 7.93 7.58 0.39 0.25Density (g/cm3) 0.015 0.012 0.019 0.025 0.051 0.041 0.041 0.034 0.6770.002 Average MW 9.9 7.9 13.2 7.1 12.5 12.4 12.4 13.3 17.0 25.0

EXAMPLE 3

Table 4 presents one embodiment of operating conditions in a series ofsynthesis reactors and process heat exchangers in the exhaust duct 16 ofa package boiler 10 as in FIG. 1. A syngas preheater 14, two ammoniasynthesis reactors 22, 24, and three BFW heaters 30, 32, 34 can bealternatingly arrayed in the exhaust duct 16 for heat rejection andrecovery. The conditions correspond to the process of Table 3 in theoperating configuration of Example 2 and FIG. 3. TABLE 4 PackageBoiler-Secondary Ammonia Converter Operation Temperature (° C.) PressureDrop Process Process Exhaust Heat Duty Fluid Exhaust Gas Fluid GasProcess Unit (MJ/h) inlet outlet inlet outlet kPa mm Hg Syngas 1,772 382413 434 423 4.1 1.15 Preheater 14 BFW Heater 9,514 207 224 423 366 7.50.59 34 Reactor 22 4,820 413 413 366 395 n/a 3.19 BFW Heater 4,304 199207 395 369 3.5 0.30 32 Reactor 24 4,504 413 413 369 396 n/a 3.20 BFWHeater 35,948 130 198 396 173 44.8  2.75 30

The invention is described above with reference to non-limiting examplesprovided for illustrative purposes only. Various modifications andchanges will become apparent to the skilled artisan in view thereof. Itis intended that all such changes and modifications within the scope andspirit of the appended claims be embraced thereby.

1. A process for converting a feed stream of nitrogen and hydrogen toammonia comprising: supplying the feed stream to an inlet of apseudoisothermal ammonia synthesis reactor having a reaction zonecomprising a plurality of catalyst tubes positioned in indirect heatexchange relationship with a gas stream in a hot gas duct of acombustion unit to maintain the reaction zone between 300° C. and 650°C.; reacting the nitrogen and hydrogen in the reaction zone to form aproduct stream having an increased ammonia content relative to the feedstream; recovering the product stream from an outlet of the reactor. 2.The process of claim 1 comprising a temperature rise between the inletand outlet less than 80° C.
 3. The process of claim 2 wherein thetemperature rises less than 55° C.
 4. The process of claim 3 wherein thereaction zone is maintained at a temperature from 370° C. to 480° C. 5.The process of claim 1, wherein the catalyst tubes have extendedsurfaces.
 6. The process of claim 1, wherein the feed stream comprisesfrom 50 to 75 volume percent hydrogen and from 25 to 40 volume percentnitrogen.
 7. The process of claim 6 wherein the feed stream comprisespurge gas from a primary ammonia synthesis loop.
 8. The process of claim7 further comprising water washing the product gas stream to removeammonia and recover an ammonia-lean stream, and injecting theammonia-lean stream upstream of a mixed feed coil in a primary reformer.9. The process of claim 8 further comprising: dehydrating the purge gasfrom the primary ammonia synthesis loop; supplying a first portion ofthe dehydrated purge gas as the feed stream to the ammonia synthesisreactor; supplying a second portion of the ammonia-lean gas stream tohydrogen recovery to obtain a hydrogen-rich stream; recycling thehydrogen-rich stream to the primary ammonia synthesis loop.
 10. Theprocess of claim 1, wherein the catalyst comprises magnetite.
 11. Theprocess of claim 1, wherein the catalyst comprises a platinum-groupmetal.
 12. The process of claim 1, wherein the combustion unit comprisesa fired heater, boiler, reformer, air preheat furnace, or gas turbine,and the hot gas comprises exhaust.
 13. The process of claim 1 whereinthe combustion unit comprises an air preheat furnace and the hot gascomprises preheated air.
 14. The process of claim 1, further comprisingheating a process stream in a heat recovery unit in communication withthe hot gas duct to contact the hot gas downstream from the ammoniasynthesis reactor.
 15. The process of claim 1 further comprisingarraying a plurality of synthesis reactors and a plurality of heatrecovery units in communication with the hot gas duct to successivelyand alternatingly indirectly reject and recover respective heat dutiesfrom synthesis reactors and heat recovery units using the hot gas ascommon heat transfer medium.
 16. A synthesis unit for convertingnitrogen and hydrogen in a feed stream to ammonia comprising: means forsupplying the feed stream to an inlet of a pseudoisothermal ammoniasynthesis reactor having a reaction zone comprising a plurality ofcatalyst tubes positioned in indirect heat exchange relationship with agas stream in a hot gas duct of a combustion unit to maintain thereaction zone between 300° C. and 650° C.; means for reacting thenitrogen and hydrogen in the reaction zone to form a product streamhaving an increased ammonia content relative to the feed stream; meansfor recovering the product stream from an outlet of the reactor.
 17. Asynthesis unit for converting nitrogen and hydrogen in a feed stream toammonia comprising: an ammonia synthesis reactor having a reaction zoneto convert the feed stream to ammonia; an inlet to introduce the feedstream into the reactor; a combustion unit having a hot gas duct incommunication with the reaction zone to contact the reaction zone with ahot gas stream to indirectly transfer heat with the reaction zone; andan outlet to discharge a product stream from the reactor.
 18. The unitof claim 17, wherein the reaction zone comprises a plurality of reactortubes and at least one catalyst.
 19. The unit of claim 18, wherein thecatalyst is selected from the group consisting of magnetite,platinum-group metals, and combinations thereof.
 20. The unit of claim17, wherein the combustion unit is selected from a fired heater, boiler,reformer, process air preheat furnace, or gas turbine.
 21. The unit ofclaim 17 further comprising an array including a plurality of ammoniasynthesis reactors and heat recovery units in communication with the hotgas duct to successively and alternatingly reject and recover heatindirectly between reactors and heat recovery units, respectively. 22.The unit of claim 17 further comprising an ammonia recovery systemconnected to the outlet including a product cooler, a product scrubber,and a scrubbing liquor stripper.
 23. A method for converting an originalammonia plant to a converted ammonia plant comprising: providing anoriginal ammonia plant comprising: a primary synthesis loop having aprimary ammonia synthesis converter for converting synthesis gas toammonia; and an ammonia recovery section for separating ammonia vaporfrom a purge gas from the primary synthesis loop comprising hydrogen,nitrogen and inerts; installing a secondary ammonia synthesis loop forreacting a feed stream to form ammonia including an ammonia synthesisreactor comprising catalyst and having a respective reaction zone incommunication with a hot gas duct of a combustion unit to contact a hotgas stream as a heat transfer medium; and diverting a portion of thepurge gas to the secondary ammonia synthesis loop to form a secondaryammonia product stream.
 24. The method of claim 23, wherein thecombustion unit comprises a fired heater, boiler, reformer, air preheatfurnace, or gas turbine.
 25. The method of claim 23, wherein thesecondary ammonia synthesis loop comprises a cooler and a condenser toseparate an ammonia-rich stream from the secondary ammonia productstream and form a residual gas stream for recycle to reformer feed.